Field Evaluations of Mechanical Ventilation Systems - 5/7/2004 - House Systems HVAC Heating Air Conditioning

Housing Performance Briefs: Field Evaluations of Mechanical Ventilation Systems

November 2001

Introduction

The use of mechanical ventilation in houses has been the focus of increasing attention in the home building industry due to concerns about the envelope tightness of new homes and indoor air quality. To provide builders and designers with more information on the use of mechanical ventilation systems the NAHB Research Center, Inc. investigated the performance of whole-house mechanical ventilation systems in new homes. Test sites were located in Alabama, Maryland, and Minnesota. The main questions that this research sought to answer were:

• Do the mechanical ventilation systems achieve the air exchange rates that they were designed to provide?
• How much do they cost to install?
• How much do they cost to operate?
• What are some pros and cons of the systems that were tested?

The Ventilation Systems:

Heat Recovery Ventilator (HRV):
A manufactured unit that transfers heat between exhaust air from the house and fresh air flow into the home. As the airstreams pass through opposite sides of the central core or heat exchanger, heat is transferred from the warmer airstream to the colder. The pre-conditioned fresh air is then delivered to the living space. The HRV also has the capacity to filter the incoming outdoor air.

Energy Recovery Ventilator (ERV):
A manufactured unit similar to the HRV except that moisture as well as sensible heat is transferred between the two airstreams.

Central Fan Integrated/Single Port Exhaust (CFI/SPE): A two-part semi-balanced mechanical ventilation system using the central HVAC blower and an independent exhaust fan. In the supply component of this system, outdoor air is ducted to one side of the central air handler and then delivered to the living space via the central ductwork using the central fan (CFI). In most cases, the duct is run to the return plenum, and outdoor air is mixed with indoor air and filtered prior to delivery. The central air handler serves as the fan for this system. Central air handler operation is provided by a controller like an AirCycler when the central system doesn’t run for heating or cooling.

In the exhaust component of this system, a separate fan and duct exhaust household air from one location in the home (SPE or single-point exhaust).

Multi-Port Exhaust (MPE):
One or more exhaust fans pull household air from several interior locations to the outside. Exhaust points may be anywhere in the home, but it is often most cost-effective for the system to provide "double-duty" as whole-house ventilation and local bath exhaust. The MPE is considered an unbalanced system because air is only intentionally exhausted - not supplied. The system depends on cracks in the building envelope for the introduction of outdoor air into the home.

Multi-Port Supply (MPS):
A multi-port supply system is similar to the multi-port exhaust system except that low volumes of outdoor air are directly supplied into the living space at several locations. With the increased pressurization indoors, household air is “exhausted” through unknown leakage paths in the building envelope.

Dehumidifying Ventilator:
A dehumidifying ventilator is a manufactured unit that offers dehumidification of incoming outdoor ventilation air prior to delivery to the home. Usually, the unit also has the capability of tempering outdoor air with indoor air and filtering the air as well.

Blending Supply Ventilator (BSV):
A variation of a multi-port supply system in which outdoor ventilation air is mixed with household air and tempered before delivery to the living space. An in-line fan draws in outdoor air and pulls house air either directly from the living space or from the return trunk of the central heating and cooling system ducts. These air streams are directly mixed, pass through a filter, and then are delivered either directly to the living space or to the supply or return plenum of the central HVAC system.

The Results

Whole-House Air Exchange Measurements
Measurements of whole house air exchange, which includes both natural infiltration and mechanical ventilation, showed that the mechanical ventilation systems achieved air exchange rates close to the design target for each site. Due to the exceptionally tight building envelopes, the houses were found to have very low air exchange rates without the use of mechanical ventilation, with an average air change rate of 0.12 ACH.

Installation Costs
The installation costs include all equipment and labor costs to the builder associated with installing the systems (including electrical and other trades). The costs do not include any design costs or cost discounts due to the complexity of installing three systems in a single home. The installation costs ranged from $399 for the Central Fan Integrated system in the 1,439 ft2 Alabama home, to $3,230 for the Energy Recovery Ventilator system in Maryland (3,721 ft2 home). The installed costs increased with 1) increases in the amount of independent ductwork, 2) the capability of ventilation equipment to independently provide heat recovery or dehumidification functions, 3) more sophisticated control systems, and 4) the size and complexity of the house.

Operating Costs
The annual cost estimates were calculated using the local utility rates for each site. Fan energy costs include energy used for both the ventilation fan and energy for the central blower if it was used for ventilation.

It should be noted that only the systems in Minnesota were required to use the central blower at all (with the exception of the CFI system in AL). Those systems in Alabama and Maryland that used the central blower for distribution of fresh air for only short periods (5 minutes per half hour) had very low annual costs associated with central blower operation.

Induced load costs represent the cost to heat or cool outdoor air brought into the house by the ventilation systems. The induced load column on the graph above provides the range of daily costs that were estimated, with the lower bound representing mild outdoor conditions and the upper bound representing summer or winter (when outdoor air conditions are more extreme). By dividing the year into mild and severe portions, this range can be used to gain a rough idea of this annual cost. For example, the BSV system in Maryland had induced load costs that ranged from $0.22 to $0.45/day. If one assumes that approximately 1/2 of the year is fairly mild ($0.22/day) and the other half is more extreme ($0.45/day), then a rough estimate of the annual induced load cost for this system is $120. When added to the fan energy cost, the total annual cost of operating this ventilation system would be roughly $230.

Other Findings
The table above offers a general comparison of the different mechanical ventilation strategies used in this project. This assessment is based upon the experiences at the three sites within this project. Additionally, the investigators applied their judgement to reflect differences in house size and complexity, labor, and climate. Findings under other conditions may differ from this summary.

De-Pressurization: Pressure differentials between zones of concern were measured under the operation of each mechanical ventilation system as well as in combination with other mechanical equipment in the homes. Since all of the homes had either sealed combustion or power vented space and water heating equipment, the garage was the primary zone of concern. None of the ventilation systems alone created significant negative pressure in the house relative to the garage under these specific conditions. At the Maryland site, which had a commercial-strength 500 cfm range hood, the house was -12 Pa relative to the garage when both the range exhaust and MPE mechanical ventilation system were operating.

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